The invention relates to a method for the in situ recovery of methane from a plurality of coal seams at the same time by forming a fissure system over and under a selected coal seam extending into a coal bearing rock strata, and recovering methane via the fissure systems and at least one borehole.
It is known that during the collapse or caving in of a void resulting from the removal of coal by mining the seam, a more or less vertical fissure system is formed in the rock strata over and under the caved in void, and the rock strata move apart to some degree under the influence of subsidence (bed-separation). In this way an extensive system of fissures is formed which may extend through unworked coal seams. Such a fissure system may reach to about 120 meters above, and about 100 meters below the caved in void (see Geologie en Mijnbouw, 41, 1962, pp. 41-44 and p. 53). The coal seems located within the range of the fissure system can thus release all or part of their adsorbed methane into the fissure system, provided that the pressure prevailing there is lower than the pressure of the methane adsorbed in the coal seams concerned.
The volume of methane that can be recovered by such a procedure is usually a multiple of the volume that has issued alone from the mined coal seam from which the fissures were initiated. This is because the fissured rock strata over and under the collapsed coal seam usually contain a plurality of additional coal seams, each of which will release a given volume of methane through the fissure system. If the geological build-up of the strata system is known, the volume of methane likely to be released through the fissure system can be accurately calculated from several parameters. The value of these parameters usually differs from one coal field to another. See, for example, Geologie en Mijnbouw, 41, 1962, pp. 55-57.
Using the Netherlands for purposes of illustration, it is known that there is a coal deposit at depths of 1000-5000 meters under substantially the whole of the Netherlands and large parts of The North Sea. This coal deposit has a thickness of at least 20 meters, measured as the joint thickness of all coal seams. The quantity of coal present within an area of, say, 100.times.100 kilometers or 10.sup.10 square meters can thus be estimated to be 2.times.10.sup.11 cubic meters. If it is assumed that an average of 10 cubic meters of methane are absorbed per cubic meter of coal, then it follows that the quantity of methane adsorbed to the coal within such area is about 2.times.10.sup.12 cubic meters. The above assumption of 10 cubic meters of methane per cubic meter of coal is reasonable, considering that in the `Peel` area about 10 cubic meters of methane have been measured per cubic meter of coal, and in South Limburg about 17 cubic meters of methane have been measured per cubic meter of coal. See the report of the Peel committee, proceedings of K.N.G.M.G. Mining series, part 5, page 83. The above calculated volume of methane is equivalent to the content of the Groningen gasfield, and thus the reserves of methane in the Netherland's sub-soil and under parts of the North Sea bottom is equal to a multiple of the reserves at Groningen, which is one of the biggest of the world. If at least a portion of this methane could be recovered, no shortage of natural gas would occur in the Netherlands for a long time to come. An equivalent calculation could be made for other coal fields in other parts of the world, except the parameters such as the total thickness of coal seams and the quantity of adsorbed methane would vary.
Methods are also known to recover gas adsorbed to coal through boreholes by increasing the permeability of the strata immediately over a coal seam. Such methods used for this purpose are generally known from the mineral oil industry and include, for example, hydraulic fracturing or hydraulic lifting of the rock overlying the said coal seam, and filling the resulting void with sand. Such a procedure has been used at Klarenthal colliery in the Saar district, see, for example, Annales des Mines de Blegique, 1, 1976, p. 25. Such methods are effective for degassing a single coal seam, but if a number of coal seems are involved, frequent repetition of the same process is required in order to recover the gas, thus entailing rather high expense.
It is suggested in Annales des Mines de Belgique, 1, 1976, pp. 25-26, that application of one additional borehole would enable methane to be recovered in an analogous manner from coal seams located in a fissure system over a coal combustion area. However, apart from needing additional boreholes, there is the possibility that the presence of such boreholes in the fissure system may cause complications during the coal combustion process. Furthermore, it is reasonable to assume that no adsorbed methane would be released at the elevated pressures in the combustion process.